1. Field of the Invention
This invention relates to an electrophotographic method for the formation of two-colored images. More particularly, it relates to a developing process for the formation of distinct images having two colors by minimizing the halo-effect.
2. Description of the Prior Art
In order to emphasize particular portions in documents, conference materials or the like printed by a color ink such as black, corrections, comments and underlines are made on the original document material or the like by the use of colored pencils (e.g., red pencils) which are distinguishable from the basic color (i.e., black) of the original. However, once the original documents or materials are copied by a copying machine to distribute to subscribers, conference members, etc., such corrections, comments and underlines are reproduced in black only, so that the corrections, comments and underlines no longer appear to be emphasized. The colors to be used for such corrections, comments or underlines do not usually have to correspond to the colors in the original, but they are required to be distinguishable from the basic color of the original.
For this purpose, various methods for the formation of two-colored images have been proposed in, for example, U.S. Pat. No. 4,189,224 and U.S. Pat. No. 4,413,899 both of which are patented to Ricoh Co. Ltd., Japan. According to these proposed methods, on a photoreceptor comprising a conductive substrate and a photoconductive layer formed on the conductive substrate, two kinds of electrostatic latent image having different polarities from each other which correspond to the two colors in the original are simultaneously formed, first. To the electrostatic latent images, two kinds of color developer charged with different polarities are adhered resulting in two-colored images, which are then subjected to a charging treatment to have the same polarity and transferred to a transfer paper followed by a fixing treatment. As the photoreceptor mentioned above, there have been two kinds of photoreceptor, one of which has a single photosensitive layer and an insulating layer on the photosensitive layer and the other of which has a photosensitive composite composed of two photosensitive layers of different spectral sensitivities. In the case where the photoreceptor is composed of a photosensitive composite, the electrostatic latent images having different polarities are formed by two charging treatments with different polarities and an exposing treatment, followed by a developing using a positively charged developer from a first developing means and a negatively charged developer from a second developing means, resulting in toner images having two colors.
In the developing step, as shown in FIGS. 5 and 6, the toner 200 from the second developing means is attached to the vicinity of the toner image 100 developed by the toner from the first developing means, resulting in a halo-image, namely a halo-effect. This phenomenon is assumed to result from electric charges located in the end portion of the electrostatic latent image on the photoreceptor. These electric charges which are charged with a different polarity from the charging polarity of the said electrostatic latent image, are induced from the ground to make toner to adhere to the surface of the photoreceptor with the electrostatic force of attraction thereof. The haloeffect is unavoidable in the formation of two-colored electrophotography as shown in FIG. 5(B), when the toner image 100 is a red color and the toner 200 located in the vicinity of the toner image 100 is a black color the red-image tends to be visually emphasized so that the halo-image with the black-colored toner is deemphasized. However, as shown in FIG. 5(A), when the toner image 200 is black and the toner 100 around the toner image 200 is red, the edge portion of the black-toner image 200 appears to be indistinguishable from the other.